Most conventional grape-picking machines use practically the same principle for harvesting grapes. This principle consists of beating or shaking the vines by imparting sinusoidal or pseudo-sinusoidal motion thereto at an amplitude and at a frequency suitable for causing the grapes or the bunches of grapes to become detached. This motion is transmitted to the vine via shaker or beater members disposed in such a manner as to act either on the vinestock or stem or else on the vegetation, i.e. on the fruit-bearing portion of the vine, depending on the type and number of shaker or beater members used. The percentage of bunches and/or individual grapes which are detached from the vines depends on the number and the amplitude of the oscillations to which a given bunch of grapes is subjected. The more energetic the shaking to which a given bunch of grapes is subjected, and the more frequently it is shaken, the more likely said bunch or its individual grapes are to become detached from the vine.
The number and the amplitude of the above oscillations depend on various parameters that may be selected, in particular on the amplitude and the frequency of the drive mechanism associated with the shaker or beater members, on the length of the active shaking zone of said shaker or beater members, on the stiffness or the flexibility thereof, and on the speed at which the machine advances, together with other factors which are imposed by the vine itself, in particular the way it is trained, its shape, and the resistance it gives to the motions of the beater or shaker members.
However, simultaneously with causing bunches of grapes and individual grapes to become detached, the shaker or beater members, when too energetically moved, may damage the vines, at least to some extent, which may be detrimental to the health of the crop. It is therefore generally necessary to find a compromise between the various above-mentioned parameters in order to obtain an acceptable percentage of harvested grapes or bunches of grapes without giving rise to unacceptable damage to the vines.
The foregoing notwithstanding, certain types of grape vines, such as the goblet-shaped vines for example, present a dense vegetation to the shaker or beater members rendering it difficult for said members to obtain a sufficient penetration in case the energetic value of the shaking or beating action would be too low, whereby the percentage of unharvested grapes would be unacceptable. Under such conditions, one of the parameters of importance is the number of the oscillations of the shaker and beater members acting on a given grape vine and therefore increasing the shaking or beating frequency could present a solution to overcome the above drawback.
Also, in case the operating speed and hence the capacity of the harvester is to be increased while still subjecting each grape vine to a given number of oscillations, the shaking or beating frequency again has to be increased.
It is experienced however that, in practice, most shaking or beating members which consist of solid rods made of a flexible material, are normally already operating at a frequency close to the resonance frequency of the rods, implying that any further increase in the operating frequency most likely would result in the rods going into resonance whereby the speed of displacement of said rods as well as the real amplitude of their motion are no longer controlled and no longer correspond to the speed and amplitude of the reciprocating drive mechanism associated with said rods. As a result, when the shaker or beater members start to "race", an uncontrollable deformation of the rods would occur, leading to damage of the vines and ultimately to the rupture of the rods.